Guard against page is None in mkdocs theme override Jinja2 templates

[Copilot]

Some mkdocs page types (e.g. in the readthedocs theme) set page to None, causing build errors when templates access page.meta.* unconditionally.

Changes

• tests/fixtures/basic_project/docs/overrides_mkdocs_theme/content.html — add page and guard before page.meta.git_revision_date_localized
• docs/howto/override-a-theme.md — update the mkdocs theme example snippet with the same guard for both git_revision_date_localized and git_created_date_localized

Before:

{% if page.meta.git_revision_date_localized %}
    <small>Last update: {{ page.meta.git_revision_date_localized }}</small>
{% endif %}

After:

{% if page and page.meta.git_revision_date_localized %}
    <small>Last update: {{ page.meta.git_revision_date_localized }}</small>
{% endif %}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Provide better documentation for customizing mkdocs theme</issue_title>
<issue_description>I think this page in the docs is a little out-of-date/doesn't provide the best information.

Updating the docs and/or directing users to https://timvink.github.io/mkdocs-git-revision-date-localized-plugin/howto/override-a-theme/#__tabbed_1_2 could be more useful as it is the more "correct" way of overriding/customizing the default mkdocs theme (or the readthedocs theme).

Additionally, customizing the "readthedocs" mkdocs theme with this snippet:

{% if page.meta.git_revision_date_localized %}
    <small>Last update: {{ page.meta.git_revision_date_localized }}</small>
{% endif %}

resulted in build errors because page is sometimes None for some page types with this theme. The minimum fix I chose was:

{% if page and page.meta.git_revision_date_localized %}
    <small>Last update: {{ page.meta.git_revision_date_localized }}</small>
{% endif %}
``` </issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Provide better documentation for customizing mkdocs theme #201

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